As used herein, the term "refrigeration system" includes mobile air conditioning systems, such as automotive, heavy trucking, agricultural, construction, and mining equipment air conditioning systems; stationary air conditioning systems; stationary refrigeration equipment, such as refrigeration and freezer containers and storage refrigerators and freezers; and building heating ventilation and air conditioning systems. A typical refrigeration system includes an evaporator, a compressor, a condenser, and an expansion device. The system may also include additional devices to enhance the functioning of the system. Commonly found devices in refrigeration systems include thermostats, pressure sensors, and switches to engage and disengage components of the system to enhance system performance and/or prevent damage due to system operation under undesirable conditions.
In a refrigeration system, a refrigerant circulates through the system. The evaporator absorbs heat from the area to be cooled, which causes the refrigerant in the evaporator to boil off into a gaseous state. The refrigerant flows from the evaporator outlet to a compressor, in which the refrigerant is pressurized to a high pressure condition. From the compressor the refrigerant circulates first to a condenser, where the refrigerant is cooled to a liquid state, and then to an expansion device, in which the pressure drops down to a low pressure. From the expansion device, the refrigerant circulates back to the evaporator, and the cycle is repeated. Efficient and safe operation of the system requires that proper refrigerant circulation and an appropriate refrigerant charge level be maintained.
It is well known that operating an air conditioning system at a low refrigerant charge condition can cause serious problems. These problems include damage to the compressor due to reduced lubricant circulation since the circulating refrigerant normally carries the lubricant. The problems also include compressor leaks or damage due to low or negative suction pressures, premature compressor clutch failure due to rapid clutch actuation, reduction in fuel economy, loss of air conditioning cooling performance, and operator annoyance. In addition, where the low charge condition is a result of an air conditioning system leak, the condition presents the problem of undesirable emission of refrigerant gases into the environment.
Historically, there have been many difficulties associated with the reliable detection of refrigerant charge levels in mobile air conditioning systems. Because of the wide range of possible operating conditions, both static and dynamic, a low charge state under a particular set of operating conditions looks identical to a full charge state under a different set of operating conditions. Therefore, even devices that appear to function in most cases will sometimes generate unacceptable false low charge alarms. Most known detection systems use a combination of two or more temperature sensors, pressure switches, or pressure transducers. Those that do not tend to be particularly unreliable.